Arc illumination apparatus and method

ABSTRACT

An illumination apparatus and method illuminates one or more reflective elements, such as solder balls on an electronic component or other protruding surfaces or objects. The illumination apparatus includes one or more arc shaped or arc shape arranged light sources that provides a substantially even illumination across the one or more reflective elements. An illumination detection device detects light beams reflecting off of the illuminated reflective elements for forming a reflected image. A method of processing the reflected image includes locating one or more points on each reflected image element representing an illuminated reflective element. The points on the reflected image elements are used to located the pattern of the reflected image elements and/or to fit an outline around each image element corresponding to a known percentage of the true dimensions of each solder ball or other reflective element. The inspection system and method thereby determines various characteristics such as the absence/presence, location, pitch, size and shape of each reflective element.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation-in-Part of U.S. patentapplication Ser. No. 08/807,397 filed Feb. 26, 1997 entitled "INSPECTIONSYSTEM" and assigned to the assignee of the present invention, now U.S.Pat. No. 5,943,125.

FIELD OF THE INVENTION

The present invention relates to illumination systems and methods and inparticular, to an illumination system and method for illuminatinggenerally spherical reflective objects.

BACKGROUND OF THE INVENTION

Digital data and signal processing techniques and technology havetremendously advanced the ability to use computers as data processingsystems to accomplish sophisticated inspection procedures without humanintervention. Almost every type of product can benefit from low cost,high precision, high speed inspection technology derived from these newdigital data and signal processing techniques.

For example, in computers and other electronic systems, the electricalconnections between electronic components ("chips") are critical to theoperation of the system. As a result of recent technological advances,electronic components are decreasing in size and increasing incomplexity, requiring a larger number of electrical connections to bemade in a smaller area. Inspection of the electronic components during amanufacturing process helps assure that electrical contacts are properlyformed and prevents failed electrical connections between electroniccomponents.

Semiconductor chips, for example, must be physically and electronicallyconnected to printed circuit boards using solder or flux betweenelectrical contacts on the chip and the circuit board. One type ofelectrical contact includes metal areas or pads on the semiconductorchip that must be electrically connected to corresponding metal areas orpads on the printed circuit board. Typically, small deposits of solderand/or flux are placed on the pads, heated and re-flowed, establishing amechanical and electrical connection between the corresponding pads.

A common soldering technique is to use preformed balls of solder thatare placed on the metal pads on the chip or substrate of an electroniccomponent, commonly known as a ball grid array (BGA). With the decreasein size of the electronic components and the increase in complexity, asmany as 400 or more solder balls must be precisely positioned in apredefined pattern on the chip or substrate to electrically connect thechip to the printed circuit board. During the process of positioning andadhering the solder balls to the metal pads on the chip or substrate, anumber of defects can occur that will detrimentally affect theelectrical connection between the chip and the printed circuit board.

If a solder ball does not sufficiently adhere to one of the pads, acritical electrical connection between the chip and the printed circuitboard could be lost. The misplacement of a solder ball can also resultin a failed connection and/or an electrical short circuit with anotheradjacent solder ball or metal pad. A solder ball that is malformed, toolarge or too small could also result in a defective electricalconnection even if properly positioned at the precise location on thepad.

Inspection of the solder balls is therefore critical to assure propersize and shape of the solder balls as well as precise placement andadherence of the solder balls to the appropriate pads on the printedcircuit board prior to establishing connections between the electroniccomponents. Inspection is also required for other electronic componentsrequiring precise electrical connections.

One prior art method of inspection is to have a human operator visuallyinspect each chip, printed circuit board or other electronic componentto detect defects in the solder balls or other electrical contacts.Manual inspection, however, is time-consuming, inaccurate, and a strainon human inspectors, particularly in light of the decreased size of theelectronic components and increased number of connections.

Video systems have also been used to inspect solder balls or othercontacts or features on electronic components. In such systems, a light,such as a ring light, illuminates the surface of the electroniccomponent to be inspected. A camera detects the light reflected from thesolder balls or contacts on the electronic component and the reflectedimage is displayed on a monitor.

The ring lights used in prior art inspection systems have been unable toprovide adequate illumination of solder balls on an electroniccomponent. One problem occurs when the ring lights do not provide lightbeams of sufficient intensity at outer regions of the area beinginspected and thus fail to illuminate some of the solder balls beinginspected, resulting in inaccurate determinations of theabsence/presence or position of the solder balls. Another problem existswhen a solder ball is only partially illuminated, preventing an accuratemeasurement of the true diameter and circularity of the solder ball.

Other inspection devices direct the light beams at a high angle withrespect to the chip, causing the light beams to reflect off themetalized pads, the substrate surface, or other substantially flatreflective surfaces that are not being inspected. In the resultingilluminated image detected by the video camera, the solder balls aredifficult to discern from the metal pads and other substantially flatreflective surfaces. This is a particular problem where the illuminatedimage is to be processed and analyzed by an image processor to detectthe absence/presence of solder balls and the condition of solder balls(e.g. location, diameter, and circularity).

Other ring lights direct light parallel to the surface of the componentbeing inspected and must be positioned against or around each electroniccomponent to obtain sufficient illumination of the entire surface of theelectronic component. If this type of ring light is not positionedagainst the surface of the component being inspected, the component willnot be sufficiently illuminated, particularly at the edges of thecomponent. This type of ring light must therefore be raised and loweredfor each individual electronic component to adequately illuminate eachelectronic component and does not allow a large number of electroniccomponents to be sequentially inspected quickly during a manufacturingprocess.

A further problem is that many prior art vision inspection systems stillrequire a human operator to examine the illuminated image of theelectronic component and detect defects such as missing, misplaced ormalformed solder balls. A visual inspection of the illuminated imagestill does not enable an accurate measurement of the size and shape ofthe solder balls.

Accordingly, a need exists for a system and method for inspecting solderballs or other reflective objects, surfaces or elements that adequatelyilluminates all of the reflective elements being inspected, allowingaccurate measurements and inspection of the reflective elements withoutilluminating other generally planar surfaces that are not beinginspected. There is also a need for a system and method that quickly andaccurately detects absence/presence of the illuminated reflectiveelements, determines their position, and measures the size and shape,e.g. the diameter and circularity of any protruding object, if desired.

SUMMARY OF THE INVENTION

The present invention features an illumination apparatus forilluminating a plurality of reflective elements disposed on a generallyplanar surface of an article to be inspected and disposed within a fieldof view of an illumination detection device. The illumination apparatuscomprises one or more arc shaped light sources or light sources arrangedin an arc configuration, for generating a plurality of light beams ontoan object to be inspected. One or more arc shaped or arc form arrangedlight sources define one or more open regions for allowing the articleto pass into the field of view within an illumination area of the arcshaped light sources. The illumination apparatus directs the pluralityof light beams generated by the arc shaped/arranged light sources towardthe article in a range of angles of illumination with respect to thegenerally planar surface of the article. The light beams directed in therange of angles of illumination provide a substantially evenillumination of the plurality of reflective elements on the article tobe inspected across the field of view such that any light beams strikingthe generally planar surface of the article are not reflected to theillumination detection device.

In one embodiment, each of the arc shaped light sources includes aplurality of light emitting elements mounted at a pre-defined angle withrespect to a plane parallel to the generally planar surface of thearticle such that a center line of each of the plurality of lightemitting elements is directed across the field of view toward anopposite edge of the article. According to this embodiment, theillumination apparatus preferably includes a light reflecting surfaceproximate the plurality of light emitting elements for reflecting atleast some of the light beams generated by the light emitting elements.A light diffusing surface is disposed proximate the light emittingelements for diffusing at least some of the light beams directed to thefield of view on the article. A pre-defined diameter formed by the oneor more arc shaped light sources is preferably in a range of about 2 1/2to 3 times a dimension of the field of view on the article to beinspected. The pre-defined angle of the light emitting elements ispreferably about 4°. According to one example, the light emittingelements are light emitting diodes (LEDs) having a beam spread of about20° to 40° and a far red spectral wavelength.

In another embodiment, the illumination apparatus includes a hightransmissivity, high diffusion diffuser, disposed proximate the arcshaped light sources, for angularly scattering the light beams generatedby the one or more arc shaped light sources and for directing the lightbeams in the range of angles of illumination. According to thisembodiment, the light source includes a plurality of light emittingelements having a center line substantially parallel to the generallyplanar surface of the article to be inspected. The high transmissivity,high diffusion diffuser, preferably includes a diffusing film having adiffuse transmission of at least 85%.

The present invention also features an inspection system for inspectinga plurality of reflective elements disposed on a generally planarsurface of an article to be inspected. The inspection system comprisesan illumination apparatus defining an aperture and including one or morearc shaped light sources for generating a plurality of light beams. Theplurality of light beams provide a substantially even illumination ofthe reflective elements on the article to be inspected and are reflectedfrom each of the plurality of reflective elements as arc shapedreflected image elements. The one or more arc shaped light sourcespreferably define one or more open regions for allowing the article topass into an illumination area within the aperture of the illuminationapparatus.

The inspection system also comprises an article support surface disposedbeneath the illumination apparatus for supporting the article such thatthe plurality of reflective elements on the article are in theillumination area within the aperture of the illumination apparatus. Anillumination detection device is disposed proximate the aperture of theillumination apparatus and has a field of view that extends through theaperture. The illumination detection device detects light beamsreflected from each of the illuminated reflective elements in the fieldof view and forms the arc shaped reflected image elements correspondingto each of the reflective elements. Light beams reflecting off thereflective elements toward the illumination detection device aredetected by the illumination detection device whereas light beamsstriking the generally planar surface of the article to be inspected arenot detected by the illumination detection device.

In one example, the illumination apparatus includes first and second arcshaped light sources defining first and second open regions. The articlesupport surface sequentially moves each of a plurality of articlesthrough the first and second open regions between the first and secondarc shaped light sources.

In one example, the article to be inspected is a ball grid array (BGA)device having a plurality of solder balls disposed on a substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will bebetter understood by reading the following detailed description, takentogether with the drawings wherein:

FIG. 1 is a schematic view of an inspection system for inspecting one ormore surfaces or objects according to the present invention;

FIG. 2 is a top view of a ring illumination apparatus used to illuminatereflective elements in the inspection system and method according to oneembodiment of the present invention;

FIG. 3 is a side cross-sectional view of the ring illumination apparatusshown in FIG. 2 taken along line III--III;

FIG. 4 is a side cross-sectional view of the ring illumination apparatusaccording to another embodiment of the present invention;

FIG. 5 is a schematic view of a reflected image detected by theinspection system according to one embodiment the present invention;

FIG. 6 is a schematic view of a single reflected image element to beprocessed according to one embodiment of the present invention;

FIG. 6A is a side schematic view of a solder ball with light beamsreflecting from the point of maximum reflection;

FIG. 7 is a top schematic view of an illumination apparatus having oneor more arc shaped light sources, according to a preferred embodiment ofthe present invention;

FIG. 8 is a top schematic view of an illumination apparatus having aplurality of straight shaped illumination light sources arranged in theshape of an arc, according to yet another embodiment of the preferredinvention;

FIG. 9 is a schematic view of a reflected image element obtained usingthe arc shaped light sources of FIGS. 8 and 9;

FIG. 10 is a flow chart of a method for processing a reflected imageaccording to the present invention;

FIG. 11 is a flow chart of a method for locating one or more points on areflected image element in a processed reflected image according to thepresent invention; and

FIG. 12 is a flow chart of a method for determining a number ofreflected image elements in a reflected image according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An inspection system 10, FIG. 1 (shown in an exaggerated perspectiveview for purposes of clarity), according to the present invention, isused to inspect one or more reflective elements, such as protrudingreflective surfaces or objects 12, disposed on an article 14. In oneexample, the protruding reflective objects 12 are disposed on agenerally planar surface 18 that has both reflective and non-reflectiveareas. The article 14 typically includes an array of protrudingreflective objects 12 made of metal or other light reflective materials.

In the exemplary embodiment, the inspection system 10 is used to inspectan array of solder balls disposed on metal pads on a chip or othersubstrate of an electronic component, such as BGA or micro BGAsemi-conductor packages, chip scale packaging (CSP), or flex circuits.The positioning, size and shape of solder balls are inspected tofacilitate proper electrical connection between the chip and otherelectronic components, such as printed circuit boards. The presentinvention, however, contemplates inspecting any type or shape ofreflective elements including, but not limited to, protrusions,deviations, and other contoured surfaces or objects on an article,arranged in any pattern on any type of article.

The present inspection system 10 includes a field of view 16 that coversthe protruding reflective objects 12 disposed on the article 14, and aring illumination apparatus 20 defining an aperture 22 through which thefield of view 16 extends. The ring illumination apparatus 20 includes asubstantially ring-shaped light source 24 that generates light beams 26and directs the light beams 26 into the field of view 16 on the article14 such that the protruding reflective objects 12 are illuminated. Thelight beams 26 preferably provide a substantially even intensity oflight across the field of view 16 on the article 14 and a substantiallyeven illumination of all of the protruding reflective objects 12 in thefield of view 16, as will be described in greater detail below. Althoughshown as a generally circular ring light, ring illumination apparatus 20may also be in the shape of an oval or other similar shape.

The inspection system 10 further includes an illumination detectiondevice 30, such as a CCD camera, disposed above the ring illuminationapparatus 20, for example, at about 14 inches above the ringillumination apparatus 20. The illumination detection device detectslight beams 32 reflected from each protruding reflective object 12 andcreates a reflected image. One example of an illumination detectiondevice 30 is a CCD camera having a resolution of about 640×480 pixels;although the present invention contemplates other types of cameras anddevices capable of detecting an illuminated image.

The substantially ring-shaped light source 24 directs light beams 26 atangles of illumination with respect to the article 14 that causenon-detected light beams 34 to reflect from the flat or planar surfaces18 on the article 14 outside of the field of view 16 or range of theillumination detection device 30. The preferred angles of illuminationof the light beams 26 are in a range of less than or equal to about 10°and are provided by different embodiments of the substantiallyring-shaped light source 24, as will be described in greater detailbelow. Accordingly, the detected light beams 32 reflecting from thereflective objects 12 create the reflected image and the non-detectedlight beams 34 reflecting from flat surfaces 18 are not included in thereflected image.

The ring illumination apparatus 20 can be mounted to a mounting support28, for example, with a mounting bracket 29. The mounting support 28 andmounting bracket 29 support the ring illumination apparatus 20 in thedesired position, allowing the article 14 to be disposed or positionedwith the reflective objects 12 in the field of view 16. An articlesupport surface 27 disposed beneath the ring illumination apparatus 20supports the article to be inspected 14 so that the reflective surfaces12 are in the field of view 16. In one example, the article supportsurface 27 is moved to index articles 14 successively into the field ofview 16 for inspection during a manufacturing process, as is well knownin the art. Alternatively, the ring illumination apparatus 20 andillumination detection device 30 are indexed or moved over each article14 being inspected. When the articles 14 and/or ring illuminationapparatus 20 and illumination detection device 30 are moved with respectto one another during inspection, the light source 24 preferably uses astrobed power supply that eliminates the effects of motion.

The inspection system 10 further includes an image processor 38 thatprocesses the reflected image and determines inspection informationincluding, but not limited to, the absence/presence, location, pitch,size, and shape of each protruding reflective surface 12, as will bedescribed in greater detail below.

The inspection system 10 optionally includes a monitor 36 that allowsthe reflected image to be viewed by an operator. The monitor 36facilitates the visual inspection and alignment of the reflectiveobjects 12 by the operator. The present invention also contemplatesother output or peripheral devices including, but not limited to, aprinter or storage device. The image processor 38 can transmit theinspection information to the monitor 36 (if provided) for viewing bythe operator or to other peripherals or devices, such as by digital I/O,RS-232 serial communication or Ethernet networking.

The preferred embodiment of the ring illumination apparatus 20, FIG. 2,includes a mounting member 40 that defines the aperture 22 and isdisposed above the article to be inspected 14. The mounting member 40preferably includes one or more fastener receiving regions 42, forbolting or otherwise fastening to mounting bracket 29. The mountingmember 40 can also include a power cord receiving region 44 thatreceives a power cord connected to a power source (not shown), forpowering the light source 24. Although the exemplary embodiment uses astrobed power supply that eliminates the effects of motion on theillumination of articles 14, the present invention contemplates any typeof power source.

The preferred embodiment of the substantially ring-shaped light source24 includes a plurality of light emitting elements 50, such as lightemitting diodes (LEDs), mounted to the mounting member 40 in asubstantially ring shape. One example of each light emitting element 50includes an LED that emits light beams having a far red spectralwavelength (e.g. about 660 nanometers) and a beam spread α ofapproximately 26° to 28°, such as a TLRA 155BP LED made by Toshiba. CCDcameras respond well to far red LED's which allow the effect of ambientlight to be filtered out and substantially eliminated during theinspection process. In the exemplary embodiment, about sixty (60) ofsuch LEDs 50 are mounted in a ring around the mounting member 40. Thepresent invention contemplates any type and number of light emittingelements that provide the desired even illumination across the field ofview.

According to one embodiment of the substantially ring-shaped lightsource 24, FIG. 3, the light beams 26 are directed at the article 14 inthe desired range of angles of illumination by mounting the lightemitting elements 50 so that a central axis or center line 54 of eachlight emitting element 50 is disposed at an angle θ with respect to aplane 52 parallel to the generally planar surface 18 of the article 14.A preferred angle θ of about 4° provides light beams 26 with a low angleof illumination (e.g. less than or equal to about 10°) onto the planarsurface 18 of the article, such that the planar surface 18 (eithernon-reflective or reflective) will either not reflect the light beams 26or will reflect the light beams 26 as non-detected light beams 34 thatextend outside field of view 16 and are therefore not detected by theillumination detection device 30. This embodiment preferably includes alight reflecting surface 64, such as white paint or a reflectivecoating, proximate to each light emitting element 50 and may alsoinclude a light diffusing surface 66 generally in front of the lightemitting elements 50, for scattering light and directing light moreevenly across the article 14.

According to known light physics principles of reflectivity, when lighthits a reflective surface, the angle of reflection is equal to the angleof incidence, measured from the axis perpendicular to the tangent of thereflective surface. If the angle θ of the light emitting element 50 istoo large, the light emitting elements 50 will provide light beams 26with a high angle of illumination, causing the light beams 26 to reflectoff the planar surfaces 18 towards the illumination detection device 30.

Lowering the mounting angle θ of the light emitting elements 50therefore lowers the angle of illumination of light beams 26 such thatlight beams 26 reflect from planar surfaces 18, such as reflectivemetalized pads and non-reflective flat surfaces, at a lower angle ofreflection outside field of view 16 as non-detected light beams 34 thatare not detected by detection device 30. The light beams 26 that hit theprotruding reflective objects 12, on the other hand, will reflectthrough the aperture 22 to the illumination detection device 30 asdetected light beams 32. The protruding reflective objects 12 arethereby illuminated for inspection while the planar surfaces 18 that arenot being inspected are not illuminated.

In this embodiment, the angle θ of the light emitting elements 50 isalso preferably greater than zero to allow sufficient spacing S betweenthe ring illumination apparatus 20 and the article 14 being inspectedwhile ensuring an even illumination across the entire field of view 16on the article 14. The spacing S of the ring illumination apparatus 20from the surface 18 allows articles 14 to be passed beneath the ringillumination apparatus 20 into and out of the field of view 16, e.g.,when inspecting during a manufacturing process. The preferred spacing Sis as small as possible without interfering with the article 14 passingbeneath the ring illumination apparatus 20 during the inspectionprocess, and typically in the range of about 1/4 to 1/2 in. The lightemitting elements 50 are also preferably positioned as close asphysically possible to the bottom region 56 of the ring illuminationapparatus 20.

The central light beam area 25 of light beam 26 directed along thecenter line 54 of each light emitting element 50 typically has thehighest power or intensity. An angle θ of approximately zero (0) degreeswill result in the central light beam area 25 being directedsubstantially parallel to the article 14. When the parallel centrallight beam area 25 is spaced from the article 14, the edges 17 of thefield of view 16 on the article 14 will only receive lower power orintensity light beams, and protruding reflective objects 12 locatedproximate to the edges 17 of the field of view 16 may not besufficiently illuminated.

By directing the central light beam area 25 towards the opposite sidesor edges 17 of the field of view 16 on the article 14, a substantiallyeven intensity of light beams is provided across the entire field ofview 16 to provide a substantially even illumination of every protrudingreflective object 12 located in the field of view 16. The angle θ oflight emitting elements 50 is therefore defined so that an imaginaryline extending from the center line 54 generally intersects or overlapsthe opposite edges or sides 17 of the field of view 16, but withoutdirecting light beams 26 at an angle of illumination high enough tocause detection of light beams reflected from the planar surfaces 18.

The substantially ring-shaped light source 24 preferably forms adiameter d, e.g., measured from the front portion of the light emittingelements 50, of approximately 2.5 to 3 times a dimension or width of thefield of view 16 on the article 14. This preferred diameter d allows thecenter light beam area 25 to be directed to the edges 17 with a lowangle of illumination while maintaining sufficient spacing S between thering illumination apparatus 20 and the article 14. In one example, adiameter of approximately 5.5 inches is used to evenly illuminate afield of view 16 on an article 14 of approximately 2 in. by 2 in.Accordingly, the diameter d of the ring-shaped light source 24 as wellas the angle θ of the light emitting elements 50 allow the light beams26 to provide even illumination across the field of view 16 on thearticle 14, while preventing illumination of unwanted planar surfaces 18and allowing articles 14 to be passed beneath the ring illuminationapparatus 20.

The preferred embodiment of the ring illumination apparatus 20 furtherincludes an upper mounting portion 60 forming the aperture 22 and a sidemounting portion 62 extending from the upper mounting portion 60. In theexemplary embodiment, the plurality of light emitting elements 50 aremounted to the side mounting portion 62 which is shaped as a ring and iswelded or otherwise secured to the upper mounting portion 60. Either thelight emitting elements 50 or the side mounting portion 62 can be angledto provide the angle θ.

The light diffusing surface 66 can be formed as a light diffusing memberor ring mounted to the upper mounting portion 60. The present inventioncontemplates other types of surfaces that diffuse or scatter the lightfrom the light source, such as a light diffusing surface directlydisposed on each individual light emitting element 50.

The light reflecting surface 64, such as white paint or other lightreflecting colors, can be provided on the side mounting portion 62 andupper mounting portion 60. In one embodiment, the diameter of theaperture 22 is approximately 70 to 80 percent the diameter d of thesubstantially ring-shaped light source 24 such that a portion 65 of theupper mounting portion 60 extends beyond the light emitting elements 50and has a light reflecting surface 64.

In accordance with another embodiment of the ring illumination apparatus20a, FIG. 4, the light beams 26 are directed at the article 14 in thedesired range of angles of illumination with a high transmissivity, highdiffusion diffuser 66a that disperses and angularly scatters light beamsas they are emitted from the light emitting elements 50a. The angularlyscattered light beams 26a provide the substantially even intensity oflight across the field of view 16 on the article 14 while preventingillumination of the flat reflective surfaces and allowing the spacing Sfrom the ring illumination apparatus 20a. In this embodiment, the angleθ of the of the light emitting elements 50 can be eliminated and ispreferably in a range of 0° to 80°.

When the high transmissivity, high diffusion diffuser 66a is used, alight reflecting surface 64 is not provided on the portion 65a of theupper mounting portion 60a that extends beyond the diffuser 66. Thisportion of the upper mounting portion 65a can have a black or othernon-reflective surface or can be eliminated entirely.

The high transmissivity, high diffusion diffuser 66a has a diffusetransmission of about 85% or more and preferably in the range of about88% to 90%. One type of high transmissivity, high diffusion diffuser 66ais an acrylic Diffusing Film Alternative (DFA) manufactured by 3M™. Thepresent invention also contemplates other suitable high transmissivity,high diffusion films that provide the desired diffuse transmission andthe desired angular scattering of the light beams.

A larger ring light apparatus can be used for larger fields of view. Fortighter applications, a smaller ring light can be used with conicalmirrors that fold the optical path to preserve the internal light pathand direct the light beams at the article in the desired angle ofillumination range.

The method of using the inspection system 10 to inspect one or morereflective elements, such as reflective objects 12, disposed on thearticle 14 includes positioning the article 14 in the field of view 16.Either a series of articles 14 are sequentially passed or indexedthrough the field of view 16 beneath the ring illumination apparatus 20or the ring illumination apparatus 20 is indexed over each article 14. Aring of light beams 26 is directed in a desired range of angles ofillumination from the ring illumination apparatus 20 toward thereflective element(s) 12 in the field of view 16 containing, to providea substantially even intensity of light beams across the field of view16 without illuminating planar surfaces 18 on the article 14.

The system and method of the present invention detects light beamsreflected from the illuminated reflective elements, such as protrudingreflective objects 12, to form a reflected image 70, FIG. 5, of theilluminated reflective elements. The reflected image includes reflectedimage elements 72 representing the illuminated reflective elements, suchas reflective objects 12. The reflected image 70 is acquired byconverting the analog output signal of the illumination detection device(camera) 30 into a plurality of digital signals, each representing asmall picture element or pixel of the image. The digital signals formingthe reflected image 70 can then be converted to analog signals fordisplay on the monitor 36 and/or processed by the image processor 38(see FIG. 1).

The reflected image 70 is processed to determine inspection informationincluding, but not limited to, absence/presence, location, size, andshape of the reflective elements. In the exemplary system and method,which is not intended to limit the present invention, the inspectionsystem 10 is used to inspect an array of solder balls disposed on solderpads on an electronic component, such as a semiconductor chip. In thereflected image 70, each solder ball appears as a reflected imageelement 72, FIG. 6, having a "doughnut" or ring shape. The inspectioninformation pertaining to the array of solder balls includes, but is notlimited to, absence/presence of each solder ball in the array, locationof each solder ball, the pitch between solder balls, malformed solderballs, the diameter of each solder ball, and the circularity of eachsolder ball.

A preferred embodiment of an illumination apparatus 90, FIG. 7,according to the present invention includes one or more arc shaped lightsources 92a, 92b defining one or more open regions 94a, 94b. In anotherembodiment, illumination apparatus 90a, FIG. 8, includes severalgenerally straight light sources 92a-92g arranged generally in the shapeof two arcs (or two (2) semicircles) defining one or more open regions94a-94b. The open regions 94a, 94b facilitate the passage of thearticles 14 into the field of view within an illumination area 99 of thearc shaped light sources 92, as indicated by arrows 95.

In general, the arc shaped or arc arranged light sources 92 generatelight beams 96 and illuminate the article 14 in the same manner as thering illumination apparatus 20 described above. The light beams 96,however, reflected from the reflective elements 12 on the article 14form a reflected image element 97 having arc shaped components 97a, 97b,FIG. 9. An array of reflected image elements 97 form a reflected imagesimilar to the reflected image 70 (FIG. 5), which can be processed todetermine inspection information.

In one example, the arc shaped light sources and the straight lightsources generally arranged in the shape of an arc 92 include a pluralityof LEDs similar to the ring illumination apparatus 20. The LEDs can bemounted on separate arc shaped mounting brackets, or can be mounted onone mounting bracket in the form of one or more arcs. Although two arcshaped light sources 92a, 92b are shown in FIG. 7, the present inventioncontemplates any number of arc shaped light sources 92 or any number ofgenerally straight or other shaped light sources 92 arranged generallyin the shape of one or more arcs.

Although the method of the present invention is described in referenceto a pattern of ring-shaped reflected image elements 72 obtained usingthe ring illumination apparatus 20. The present invention also featuresa method 100, FIG. 10, of processing a reflected image 70 including apattern of reflected image elements 72. The method also applies to apattern of reflected image elements 97 having arc shaped components 97a,97b obtained using the arc shaped or arc arranged light source(s) 92.

The method of processing the reflected image includes locating thepattern of reflected image elements 72 in the reflected image 70, step110; fitting an outline or mid point line to each reflected imageelement 72 in the pattern of reflected image elements, step 120; anddetermining inspection information pertaining to the reflective elementsrepresented by the reflected image 70, step 130.

One way of locating the pattern of reflected image elements 72, step110, is by identifying a group of reflected image elements 72, forexample, by creating a window 75 (FIG. 5) around an outside row or groupof reflected image elements 72. A point on each reflected image element72 in the group, such as an outside edge 76, is then located. One ormore lines 74 are fit to the outside edges 76 or other point on thereflected image elements 72 (see FIG. 5) to locate the pattern anddetermine the "expected" location of each reflected image element 72 inthe pattern. Although the exemplary embodiment shows a rectangular gridof image elements 72, the present invention also contemplates locating acircular or other pattern of reflected image elements. In a circularpattern, the outside edges of the image elements formed in a circle arelocated and a circle fit algorithm is used to fit a circle to theoutside edges and to locate the pattern.

Another way of locating the reflected image is by locating referencemarks or fiducials 73 disposed in predetermined locations with respectto the reflected image elements 72 in the reflected image 70. A furtherway of locating the reflected image is by correlating a template ormodel of a known pattern with the reflected image.

Once the pattern of the reflected image 70 is located, the outline ormid point line is fit to each reflected image element 72, 97, step 120,for example, by creating a window 78 (FIGS. 6 and 9) around eachreflected image element 72, 97 at each expected location and locatingmultiple points 76a-76d on the edge 76 of the reflected image element72, 97 or locating multiple midpoints 71a-71d within the reflected imageelement 72, 97. For a reflected image element 72, 97 having a circularor arc shape, at least three of the edge points 76a-76d or midpoints71a-71d are needed to fit the respective circular outline 77a, or midpoint line 77b. Four edge points 76a-76d or midpoints 71a-71d are neededto determine the circularity of the respective outline 77a, or mid pointline 77b. The edge points 76a-76d or midpoints 71a-71d and respectiveoutlines or mid point lines 77a, 77b fit to the image element(s) 72, 97correspond to a known percentage of the true the diameter, circularity,or other dimension of the solder ball or other reflective element beingmeasured, as described in greater detail below. In the preferredembodiment, eight (8) or more points are located, and the locations ofthe points are fed to a circle fit algorithm which accurately determinesthe size and circularity of the reflective elements.

The method 200, FIG. 11, of locating the points 76a-76d on the outsideedge 76 or the midpoints 71a-71d of each reflected image element 72, 97includes dividing the reflected image 70 into a plurality of pixelshaving a gray scale value corresponding to an intensity level ofreflected light in the reflected image 70, step 210. In one example,each pixel is represented by eight (8) bits with a gray scale value ofzero (0) being the darkest pixel and a gray scale value of 255 being thebrightest pixel. Vectors 79a-79d (FIGS. 6 and 9) are positioned tointersect the expected location of the image element 72, 97, step 220.

A series of pixels along each vector 79a-79d is examined to find edgepoints 76a-76d or midpoints 71a-71d, step 230, along the vectors79a-79b. Although shown as radial vectors, the vectors can be positionedin various configurations, such as a cross-hatched configuration,provided that the vectors intersect the image element 72, 97. The edgepoints 76a-76d or midpoints 71a-71d are then used to locate the image byfitting the line 74 or to determine a diameter, circularity or otherdimension of the,solder ball by fitting the outline 77a, or mid pointline 77b, as describe above.

To locate the edge points 76a-76d along the path of each vector 79a-79d,an intensity gradient at each pixel is determined by differentiatingbetween the gray scale values of pixels on either side of each pixel.The point of maximum gradient (i.e. the steepest or greatest change fromdarkest to brightest pixels) is located and is assigned to correspond tothe edge 76 of the reflected image element 72, 97.

The preferred method includes taking a group of points having thehighest intensity gradients for each reflected image element 72, 97 andfitting an ellipse to the highest intensity gradients. The peak of theellipse corresponds to the point of highest intensity gradient withsub-pixel accuracy, thereby allowing a more accurate calculation of thedimensions and shape of the solder ball or other reflective surfacerepresented by each reflected image element 72, 97. The edge points76a-76d and circular outline 77a fit to the edge points 76a-76dcorrespond to a known percentage of the true dimensions of the solderball being measured, e.g. about 70%.

To locate the midpoints 71a-71d along the vectors 79a-79d, a gray scalevalue at each pixel is determined and the midpoint 71 in the ring shapedimage element 72, 97 corresponds to the pixel having the highest grayscale value. The preferred method includes taking a group of pixelshaving the highest gray scale value, e.g. the brightest pixel and one ormore adjacent pixels, and fitting an ellipse to these pixels. The peakof the ellipse corresponds to the midpoint 71 having the highestintensity with sub-pixel accuracy.

The brightest midpoints 71a-71d and the circular outline 77b fit tomidpoints 71a-71d correspond to a known dimension of the solder ballbeing measured. Each solder ball 80, FIG. 6A, has point 82 of maximumreflection located on the spherical surface of the solder ball 82 atabout 45° from the top that reflects light beams 84 of the highestintensity. The location of the point 82 of maximum reflection is equalto sin(45°) (or 0.7071) times the diameter of the solder ball 80. Thebrightest midpoints 71a-71d and the circular mid point line 77btherefore correspond to the point(s) 82 of maximum reflection and theknown percentage of the true diameter of the solder ball 80. Using thebrightest midpoint 71a-71d of the image element 72, 97 provides a truermeasurement than using the intensity gradient method to locate the edgepoints 76a-76d of the image element 72, 97 if the edges 76 of the imageelements 72, 97 are not focused.

Since the method above processes the reflected image elements 72according to their expected locations as determined by fitting the lines74 along the edges 76 (FIG. 5), an extra or additional reflected imageelement 72a corresponding to an added solder ball or other reflectivesurface may not be detected.

The present image processing method further includes a method 300, FIG.12, of determining the number of reflected image elements 72, 97.Determining the number of reflected image elements 72 in the entirereflected image 70, not just at the expected locations, allows theabsence/presence of solder balls or other reflective surfaces to beeasily determined.

The number of reflected image elements 72, 97 is determined by dividingthe reflected image 70 into a plurality of pixels having a gray scalevalue corresponding to the intensity of light in the reflected image 70,step 310. Groups of pixels having a gray scale value above a thresholdvalue are then located, step 320, and the number of groups of pixels(corresponding to the number of reflected image elements 72, 97) arecounted, step 330. Determining the number of reflected image elements72, 97 allows a determination of missing, misplaced or extra reflectiveelements, such as solder balls or other reflective objects.

The image processing method of the present invention also includes acalibration process that can be performed to allow the inspectioninformation measurements to be expressed in conventional units and tocorrect for magnification, perspective errors, and other effects. Thecalibration procedure involves measuring a target of known dimensions,for example, an array of dots having known sizes and known locations onthe target. The relationship between the coordinates of the target imageas determined by the image processor and the known location of the dotson the target are calculated to determine the correlation between pixelsand conventional units. The present method also contemplates heightcorrection of the part being inspected relative to the calibrationtarget to account for errors caused by optical magnification.

The present method for processing the reflected image 70 used togetherwith the ring illumination apparatus 20 or arc shaped light sources 92a,92b described above provides a more accurate determination of thediameter and circularity of solder balls on an electronic component orother article. Directing the light beams in the desired range of anglesof illumination, e.g. by angling the light source 24 at an angle θ, byusing a high diffusion diffuser, or by using conical mirrors,illuminates a portion of the top surface of each solder ball so that thepoint of maximum gradient or highest gray scale value corresponds to aknown percentage of the true diameter of the solder ball. Directing thelight beams in the desired range of angles of illumination, alsoprevents illumination of solder pads or other generally planar surfaceareas from interfering with the determination of the maximum gradient.When the illumination apparatus of the present invention is usedtogether with the present method of processing the reflected image, themeasurements made during the inspection have a high degree of accuracyand repeatability. The present invention, however, contemplates usingthe ring illumination apparatus or arc shaped light sources with othermethods for processing the reflected image as well as using this methodof processing the reflected image with another type of illuminationapparatus.

Accordingly, the present invention provides an inspection system andmethod that accurately inspects and measures one or more surfaces orobjects, such as solder balls, protrusions, intrusions, deviations,concavities, and other reflective elements on an article. The inspectionsystem and method evenly illuminates all of the surfaces or objects tobe inspected without concern for illuminating flat or unwanted surfaceson the article or electronic component. The inspection method alsoincludes an image processing method that more accurately measures thesize and shape of individual reflective elements or solder balls.

Modifications and substitutions by one of ordinary skill in the art areconsidered to be within the scope of the present invention which is notto be limited except by the claims which follow.

What is claimed is:
 1. An illumination apparatus, for illuminating aplurality of reflective elements disposed on a generally planar surfaceof an article to be inspected and disposed within in a field of view ofan illumination detection device, said illumination apparatuscomprising:at least one arc shaped light source, for generating aplurality of light beams, said at least one arc shaped light sourcedefining at least one open region, for allowing said article to passinto said field of view within an illumination area of said at least onearc shaped light source; means for directing said plurality of lightbeams generated by said at least one arc shaped light source toward saidarticle in said field of view in a range of angles of illumination withrespect to said generally planar surface of said article, wherein saidlight beams directed in said range of angles of illumination provide asubstantially even illumination of said plurality of reflective elementson said article to be inspected across said field of view such that anyof said plurality of light beams striking said generally planar surfaceof said article to be inspected are not reflected to said illuminationdetection device.
 2. The illumination apparatus of claim 1 wherein saidat least one arc shaped light source includes first and second arcshaped light sources defining first and second open regions, forallowing said article to pass into said field of view within saidillumination area between said first and second arc-shaped lightsources.
 3. The illumination apparatus of claim 1 wherein said at leastone arc shaped light source includes a plurality of light emittingelements, and wherein said means for directing said plurality of lightbeams in said range of angles of illumination includes said plurality oflight emitting elements mounted at a predefined angle with respect to aplane parallel to said generally planar surface of said article suchthat a centerline of each of said plurality of light emitting elementsis directed across said field of view toward an opposite edge of saidarticle.
 4. The illumination apparatus of claim 3 further including alight reflecting surface proximate to said plurality of light emittingelements, for reflecting at least some of said light beams generated bysaid light emitting elements.
 5. The illumination apparatus of claim 4further including a light diffusing surface proximate to said pluralityof light emitting elements.
 6. The illumination apparatus of claim 3wherein a predefined diameter of said at least one of arc shaped lightsource is in a range of about 2 1/2 to 3 times a dimension of said fieldof view on said article to be inspected.
 7. The illumination apparatusof claim 3 wherein said plurality of light emitting elements are lightemitting diodes (LEDs).
 8. The illumination apparatus of claim 7 whereineach of said plurality of LEDs generates a light beam which spreads in arange of about 20 to 40°.
 9. The illumination apparatus of claim 8wherein each of said plurality of LEDs emits light beams having a farred spectral wavelength.
 10. The illumination apparatus of claim 1wherein said means for directing said plurality of light beams in saidrange of angles of illumination includes a high transmissivity, highdiffusion diffuser, disposed proximate to said at least one arc shapedlight source, for angularly scattering said plurality of light beamsgenerated by said at least one arc shaped light source.
 11. Theillumination apparatus of claim 10 wherein said at least one arc shapedlight source includes a plurality of light emitting elements, andwherein a centerline of each of said light emitting elements issubstantially parallel to said generally planar surface of said articleto be inspected.
 12. The illumination apparatus of claim 11 wherein saidhigh transmissivity, high diffusion diffuser includes a diffusing filmhaving a diffuse transmission of at least 85%.
 13. The illuminationapparatus of claim 1 wherein said plurality of reflective elementsincludes an array of solder balls disposed on an electronic component.14. The illumination apparatus of claim 1 wherein said at least one arcshaped light source includes a plurality of generally straight lightsources arranged generally in the shape of first and second arcs, saidfirst and second arcs defining first and second open regions, forallowing said article to pass into said field of view within saidillumination area between said first and second arc-shaped lightsources.
 15. A illumination apparatus, for illuminating a plurality ofreflective elements disposed on a generally planar surface of an articleto be inspected and disposed within in a field of view of anillumination detection device, said illumination apparatus comprising:atleast one arc shaped light source, for generating a plurality of lightbeams; and a high transmissivity, high diffusion diffuser, disposedproximate to said at least one arc shaped light source, for angularlyscattering said plurality of light beams generated by said at least onearc shaped light source, and for directing said plurality of light beamsgenerated by said at least one arc shaped light source toward saidarticle in said field of view in a range of angles of illumination withrespect to said generally planar surface of said article, wherein saidlight beams directed in said range of angles of illumination provide asubstantially even illumination of said plurality of reflective elementson said article to be inspected across said field of view such that anyof said plurality of light beams striking said generally planar surfaceof said article to be inspected are not reflected to said illuminationdetection device.
 16. An inspection system for inspecting a plurality ofreflective elements disposed on a generally planar surface of an articleto be inspected, said inspection system comprising:an illuminationapparatus defining an aperture, said illumination apparatus including atleast one arc shaped light source, for generating a plurality of lightbeams, wherein said plurality of light beams provide a substantiallyeven illumination of said plurality of reflective elements on saidarticle to be inspected and are reflected from each of said plurality ofreflective elements as at least one arc shaped reflected image element,said at least one arc shaped light source defining at least one openregion, for allowing said article to pass into an illumination areawithin said aperture of said illumination apparatus; an article supportsurface, disposed beneath said illumination apparatus, for supportingsaid article such that said plurality of reflective elements on saidarticle are in said illumination area within said aperture of saidillumination apparatus; and an illumination detection device disposedproximate to said aperture of said illumination apparatus and having afield of view, said illumination detection device detecting light beamsreflected from each illuminated reflective element in said field ofview, and forming said arc shaped reflected image elements correspondingto each of said plurality of reflective elements, such that light beamsreflecting off said each illuminated reflective element toward saidillumination detection device are detected by said illuminationdetection device and light beams striking said generally planar surfaceof said article to be inspected are not detected by said illuminationdetection device.
 17. The inspection system of claim 16 wherein said atleast one arc shaped light source includes first and second arc shapedlight sources defining first and second open regions between said firstand second arc shaped light sources.
 18. The inspection system of claim17 wherein said article support surface sequentially moves a pluralityof said articles through said first and second open regions between saidfirst and second arc shaped light sources.
 19. A illumination apparatus,for illuminating a plurality of reflective elements disposed on agenerally planar surface of an article to be inspected and disposedwithin in a field of view of an illumination detection device, saidillumination apparatus comprising:at least one arc shaped light sourceincluding a plurality of light emitting elements arranged to form anarc, for generating a plurality of light beams, each of said pluralityof light emitting elements being mounted at a predefined angle withrespect to said generally planar surface of said article such that saidplurality of light beams are directed in a range of angles ofillumination; a light reflecting surface disposed generally behind eachof said plurality of light emitting elements, for reflecting at leastsome of said light beams generated by said light emitting elements; alight diffusing surface disposed generally in front of each of saidplurality of light emitting elements, for diffusing at least some ofsaid plurality of light beams as said plurality of light beams aredirected toward said article within said field of view; and wherein saidplurality of light beams are directed toward said article in said fieldof view in said range of angles of illumination with respect to saidgenerally planar surface of said article such that said plurality ofreflective elements on said article are illuminated simultaneously andany of said plurality of light beams striking said generally planarsurface of said article to be inspected are not reflected to saidillumination detection device.
 20. The illumination apparatus of claim19 wherein said light reflecting surface includes a white surfaceproximate to said light emitting elements.
 21. The illuminationapparatus of claim 19 wherein said plurality of light emitting elementsare light emitting diodes (LEDs).
 22. The illumination apparatus ofclaim 21 wherein each of said plurality of LEDs generates a light beamwhich spreads in a range of about 20° to 40 .
 23. The illuminationapparatus of claim 21 wherein each of said plurality of LEDs emits lightbeams having a far red spectral wavelength.
 24. The illuminationapparatus of claim 19 wherein said predefined angle of said lightemitting elements is about 4°.
 25. The illumination apparatus of claim19 wherein a diameter of said at least one arc shaped light source is ina range of about 2 1/2 to 3 times a dimension of said field of view onsaid article to be inspected.
 26. The illumination apparatus of claim 19wherein said plurality of reflective elements include an array of solderballs disposed on an electronic component.
 27. The illuminationapparatus of claim 19 wherein a central light beam area of each of saidplurality of light beams is directed across said field of view toward anopposite edge of said field of view on said article to be inspected. 28.A illumination apparatus, for illuminating a plurality of reflectiveelements disposed on a generally planar surface of an article to beinspected and disposed within in a field of view of an illuminationdetection device, said illumination apparatus comprising:at least onearc shaped light source having a predefined diameter, said at least onearc shaped light source including:a plurality of light emittingelements, mounted at a predefined angle with respect to a plane parallelto said generally planar surface of said article, for generating anddirecting a plurality of light beams toward said article in said fieldof view, wherein said predefined diameter and said predefined angle aredefined such that a central light beam area of each of said plurality oflight beams is directed across said field of view toward an oppositeedge of said field of view on said article to be inspected such thatsaid plurality of light beams provide a substantially even illuminationof each of said plurality of reflective elements in said field of viewon said article to be inspected without being reflected from saidgenerally planar surface to said illumination detection device.
 29. Theillumination apparatus of claim 28 wherein said predefined angle isabout 40°.
 30. The illumination apparatus of claim 29 wherein saidpredefined diameter is about 2 1/2 to 3 times a dimension of said fieldof view on said article to be inspected.
 31. The illumination apparatusof claim 28 further including a light reflecting surface disposedgenerally behind each of said plurality of light emitting elements, forreflecting at least some of said light beams generated by said lightemitting elements.
 32. The illumination apparatus of claim 31 furtherincluding a light diffusing surface disposed generally in front of eachof said plurality of light emitting elements, for diffusing at leastsome of said plurality of light beams as said plurality of light beamsare directed towards said article within said field of view.
 33. Theillumination apparatus of claim 28 wherein said plurality of lightemitting elements are light emitting diodes (LEDs).
 34. The illuminationapparatus of claim 33 wherein each of said plurality of LEDs generates alight beam which spreads in a range of about 20° to 40°.
 35. Theillumination apparatus of claim 34 wherein each of said plurality ofLEDs emits light beams having a far red spectral wavelength.